From patchwork Tue Jul 30 06:46:57 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Pankaj Gupta X-Patchwork-Id: 11065113 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 94CF2746 for ; Tue, 30 Jul 2019 06:47:56 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 842E5285EA for ; Tue, 30 Jul 2019 06:47:56 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 7796F2862A; Tue, 30 Jul 2019 06:47:56 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 6A358285EA for ; Tue, 30 Jul 2019 06:47:55 +0000 (UTC) Received: from localhost ([::1]:58336 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hsLvO-0000ap-Gd for patchwork-qemu-devel@patchwork.kernel.org; Tue, 30 Jul 2019 02:47:54 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:33000) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hsLux-0008Ci-3B for qemu-devel@nongnu.org; Tue, 30 Jul 2019 02:47:28 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hsLuu-0008Ne-S4 for qemu-devel@nongnu.org; Tue, 30 Jul 2019 02:47:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41276) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hsLuu-0008M5-JY for qemu-devel@nongnu.org; Tue, 30 Jul 2019 02:47:24 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9E6BB81DE3; Tue, 30 Jul 2019 06:47:21 +0000 (UTC) Received: from dhcp201-121.englab.pnq.redhat.com (ovpn-116-177.sin2.redhat.com [10.67.116.177]) by smtp.corp.redhat.com (Postfix) with ESMTP id 73BB0600CD; Tue, 30 Jul 2019 06:47:01 +0000 (UTC) From: Pankaj Gupta To: qemu-devel@nongnu.org Date: Tue, 30 Jul 2019 12:16:57 +0530 Message-Id: <20190730064658.27369-1-pagupta@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Tue, 30 Jul 2019 06:47:21 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH] virtio pmem: user document X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: aarcange@redhat.com, pagupta@redhat.com, cohuck@redhat.com, david@redhat.com, mst@redhat.com, lcapitulino@redhat.com, stefanha@redhat.com, riel@surriel.com, nilal@redhat.com Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP This patch documents the steps to use virtio pmem. It also documents other useful information about virtio pmem e.g use-case, comparison with Qemu NVDIMM backend and current limitations. Signed-off-by: Pankaj Gupta --- docs/virtio-pmem.txt | 65 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) create mode 100644 docs/virtio-pmem.txt diff --git a/docs/virtio-pmem.txt b/docs/virtio-pmem.txt new file mode 100644 index 0000000000..fc61eebb20 --- /dev/null +++ b/docs/virtio-pmem.txt @@ -0,0 +1,65 @@ + +QEMU virtio pmem +=================== + + This document explains the usage of virtio pmem device + which is available since QEMU v4.1.0. + + The virtio pmem is paravirtualized persistent memory device + on regular(non-NVDIMM) storage. + +Usecase +-------- + Allows to bypass the guest page cache and directly use host page cache. + This reduces guest memory footprint as host can make efficient memory + reclaim decisions under memory pressure. + +o How does virtio-pmem compare to the nvdimm emulation supported by QEMU? + + NVDIMM emulation on regular(non-NVDIMM) host storage does not persists + the guest writes as there are no defined semantecs in the device specification. + With virtio pmem device, guest write persistence on non-NVDIMM storage is + supported. + +virtio pmem usage +----------------- + virtio pmem device is created with a memory-backend-file with the below + options: + + -machine pc -m 8G,slots=$N,maxmem=$MAX_SIZE + -object memory-backend-file,id=mem1,share,mem-path=$PATH,size=$SIZE + -device virtio-pmem-pci,memdev=mem1,id=nv1 + + where: + - "object memory-backend-file,id=mem1,share,mem-path=$PATH,size=$VIRTIO_PMEM_SIZE" + creates a backend storage of size $SIZE on a file $PATH. All + accesses to the virtio pmem device go to the file $PATH. + + - "device virtio-pmem-pci,id=nvdimm1,memdev=mem1" creates a virtio pmem + device whose storage is provided by above memory backend device. + + Multiple virtio pmem devices can be created if multiple pairs of "-object" + and "-device" are provided. + +Hotplug +------- +Accomplished by two monitor commands "object_add" and "device_add". + +For example, the following commands add another 4GB virtio pmem device to +the guest: + + (qemu) object_add memory-backend-file,id=mem2,share=on,mem-path=virtio_pmem2.img,size=4G + (qemu) device_add virtio-pmem-pci,id=virtio_pmem2,memdev=mem2 + +Guest Data Persistence +---------------------- +Guest data persistence on non-NVDIMM requires guest userspace application to +perform fsync/msync. This is different than real nvdimm backend where no additional +fsync/msync is required for data persistence. + +Limitations +------------ +- Real nvdimm device backend is not supported. +- virtio pmem hotunplug is not supported. +- ACPI NVDIMM features like regions/namespaces are not supported. +- ndctl command is not supported.